Xid in log_line_prefix

Patches,

This trivial patch adds the transaction Id to the list of escapes
available to log_line_prefix.  (Note that I used the top transaction Id,
not the current subtransaction Id, which is clearly useless for PITR
purposes.)

One important question is what letter to use for this.  It seemed to me
that the most obvious letter was "x".  So I used that, and changed the
"postmaster and friends stop here" to "q".

I also thought of using %t, but that is already taken too.

Anyone has a better idea?

--
Alvaro Herrera (<alvherre[a]dcc.uchile.cl>)
Bob [Floyd] used to say that he was planning to get a Ph.D. by the "green
stamp method," namely by saving envelopes addressed to him as 'Dr. Floyd'.
After collecting 500 such letters, he mused, a university somewhere in
Arizona would probably grant him a degree.              (Don Knuth)